A new study of a cyberattack last month against Ukrainian power companies suggests malware didn't directly cause the outages that affected at least 80,000 customers.

Instead, the malware provided a foothold for key access to networks that allowed the hackers to then open circuit breakers that cut power, according to information published Saturday by the SANS Industrial Control Systems (ICS) team.

Experts have warned for years that industrial control systems used by utilities are vulnerable to cyberattacks. The Dec. 23 attacks in Ukraine are the most prominent example yet of those fears coming to fruition.

SANS ICS said the attacks demonstrated planning and coordination. Tensions between Ukraine and Russia have been high since Russia annexed Crimea in 2014.

While malware was used to gain access to networks, the attackers also used direct intervention to try to mask their actions to the power systems operators, SANS ICS said.

They also conducted denial-of-service attacks on the utilities' phone systems to block complaints from affected customers, the organization said.

The attacks reportedly affected two service providers -- Prykarpattyaoblenergo and Kyivoblenergo, the latter of which said in a service update that 80,000 customers after 30 substations went offline, SANS ICS said.

"Simply put, there is still evidence that has yet to be uncovered that may refute the minutia of the specific components of the malware portion of the attack," wrote Michael Assante, director of SANS ICS.

KillDisk aims to make a computer unusable by overwriting the Master Boot Record (MBR), the first sector of a PC’s hard drive that the computer looks to before loading the operating system. It can also overwrite files with junk data, according to Symantec.

Assante wrote that KillDisk wouldn't have been compatible with the type of SCADA (supervisory control and data acquisition) systems used by utilities. But it may have been employed to wipe other files that would have helped to restore systems.

The power companies appear to have restored services by manually closing the circuit breakers within three and six hours, Assante said.

"In many ways, the Ukrainian operators should be commended for their diligence and restoration efforts," he wrote.